Rechargeable batteries are used in a variety of devices. For example, they may be used in small, handheld, devices, such as mobile phones, laptops, etc. In addition, rechargeable batteries may be used in larger devices, for example battery powered vehicles such as electric scooters and electric wheelchairs.
Wherever rechargeable batteries are used, it is common to provide a charger connection point for connecting a charger to the device in which the rechargeable battery is used, to enable the rechargeable battery to be recharged. At least a portion of the wiring from the charger to the battery will be provided externally to the device, and accordingly can be vulnerable to damage. For example, considering a simple device where the charger is connected via a charging lead directly to a charger connection point in the housing of the device, then the charging lead itself is potentially vulnerable to damage. Considering the example of a battery powered vehicle, then it is often not only the charging lead from the charger to the charger connection point that is potentially vulnerable to damage, but in addition there may be at least one lead on the battery powered vehicle itself passing from the charger connection point to the battery or some intermediate unit, that may also be vulnerable to damage.
For example, within a battery powered vehicle, it is common for multiple separate circuits to be driven from the rechargeable battery under the control of a power control system. A separate cable spur may be provided from the power control system to the charger connection point and at least part of that separate cable spur may run along an exposed surface of the vehicle. In an alternative embodiment, a common power line may be run from the power control system to a control input module which also incorporates the charger connection point. Hence, charging may occur via this common power line, but in addition power can be drawn down this common power line to power the control input module. Again, this common power line may run at least partially along an external surface of the vehicle. As a specific example, the control input module may take the form of a joystick mounted, for example, on an armrest of an electric wheelchair, and in that instance the common power line may run along the underside of the armrest and then be routed onward to the power control system within the wheelchair (typically mounted somewhere underneath the seat of the wheelchair).
Irrespective of how the wiring is provided from the charger to the battery, or to an intermediate unit such as a power control system (this wiring being referred to hereafter as the “charger cable”), that charger cable needs to be able to allow charging currents to be passed through the wires in order to charge the battery. For battery powered vehicles, the charging current may be in the magnitude of 10 amps. It is common to provide a current tripping element along the current charging path in order to limit the charging current, and for example such a current tripping element may be formed by one or more solid-state self-resetting fuses. Such solid-state self-resetting fuses are very reliable and have a high thermal mass that allows higher peak currents for shorter periods thereby avoiding false trips.
However, it is possible for the charger cable to become damaged and for shorts to occur between the positive and negative supply lines in the cable. The shorting can be caused for a variety of reasons, for example due to damaged cable insulation caused by vibration and abrasion, or by direct mechanical action as can be common in mobile vehicles. A direct short circuit will cause enough current out of the batteries for the self-resetting protection fuses to operate rapidly and prevent excess current flow. However, if the short is where the wires only just touch together, or short intermittently, the high initial surge of current available before the fuses trip can, under certain conditions, cause the wires to melt at the point of contact causing a high temperature plasma arc whose behaviour is difficult to predict.
Rarely, following repeated intermittent shorting, a self-sustaining arc may occur that has enough resistance to keep the current below the trip threshold of the protection fuses. For example, as mentioned earlier, in mobile vehicles, these protection fuses may allow current up to 10 amps to flow without tripping. This arc could potentially lead to a highly undesirable cable loom fire.
One possible way to prevent the above problem would be to provide a simple diode within the charger cable. Positive charging currents would then be allowed, but reverse currents from the battery would be inhibited. However, one problem with this solution is that the forward voltage drop of the diode will cause a significant amount of local heat dissipation to occur, which is undesirable. Further, modern intelligent battery chargers often need to draw some reverse current from the battery in order to read the battery voltage correctly. Furthermore, in some devices, for example battery powered vehicles, the charger cable forms a shared power line with other circuits, for example a control input module, and those other circuits need to be able to draw current from the battery (in the opposite direction to the current flow that occurs during charging) in order to power those circuits.
Accordingly, it would be desirable to provide an improved technique for protecting against the shorting problems that can occur when a charger cable becomes damaged.